Cardiac rhythm management systems and methods using multiple morphology templates for discriminating between rhythms

ABSTRACT

This document describes systems, devices, and methods that use multiple morphology templates for discriminating between rhythms, such as supraventricular tachyarrhythmias (SVTs) and ventricular tachyarrhythmias (VTs), for delivering a countershock in response to a VT episode, but withholding delivery of such a countershock in response to an SVT episode. In certain examples, the particular morphology used for storing morphological features is selected at least in part using a sensor-indicated activity level of a subject, or a metabolic need of the subject.

CROSS REFERNCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/291,200, filed on Nov. 8, 2002, now U.S. Pat. No. 7,031,764, thespecification of which is incorporated herein by reference.

TECHNICAL FIELD

This document relates generally to medical systems, devices, andmethods, and particularly, but not by way of limitation, to cardiacrhythm management systems and methods using multiple templates fordiscriminating between rhythms.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm. Its sinoatrial node generates intrinsic electrical cardiacsignals that depolarize the atria, causing atrial heart contractions.Its atrioventricular node then passes the intrinsic cardiac signal todepolarize the ventricles, causing ventricular heart contractions. Theseintrinsic cardiac signals can be sensed on a surface electrocardiogram(i.e., a “surface ECG signal”) obtained from electrodes placed on thepatient's skin, or from electrodes implanted within the patient's body(i.e., an “electrogram signal”). The surface ECG and electrogramwaveforms, for example, include artifacts associated with atrialdepolarizations (“P-waves”) and those associated with ventriculardepolarizations (“QRS complexes”).

A normal heart is capable of pumping adequate blood throughout thebody's circulatory system. However, some people have irregular cardiacrhythms, referred to as cardiac arrhythmias. Moreover, some patientshave poor spatial coordination of heart contractions. In either case,diminished blood circulation may result. For such patients, a cardiacrhythm management system may be used to improve the rhythm and/orspatial coordination of heart contractions. Such systems are oftenimplanted in the patient and deliver therapy to the heart.

Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular lead wire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly. Such pacers may also coordinate atrial andventricular contractions to improve pumping efficiency.

Cardiac rhythm management systems also include cardiac resynchronizationtherapy (CRT) devices for coordinating the spatial nature of heartdepolarizations for improving pumping efficiency. For example, a CRTdevice may deliver appropriately timed pace pulses to differentlocations of the same heart chamber to better coordinate the contractionof that heart chamber, or the CRT device may deliver appropriately timedpace pulses to different heart chambers to improve the manner in whichthese different heart chambers contract together.

Cardiac rhythm management systems also include defibrillators that arecapable of delivering higher energy electrical stimuli to the heart.Such defibrillators include cardioverters, which synchronize thedelivery of such stimuli to sensed intrinsic heart activity signals.Defibrillators are often used to treat patients with tachyarrhythmias,that is, hearts that beat too quickly. Such too-fast heart rhythms alsocause diminished blood circulation because the heart isn't allowedsufficient time to fill with blood before contracting to expel theblood. Such pumping by the heart is inefficient. A defibrillator iscapable of delivering a high energy electrical stimulus that issometimes referred to as a defibrillation countershock, also referred tosimply as a “shock.” The countershock interrupts the tachyarrhythmia,allowing the heart to reestablish a normal rhythm for the efficientpumping of blood. In addition to pacers, CRT devices, anddefibrillators, cardiac rhythm management systems also include devicesthat combine these functions, as well as monitors, drug deliverydevices, and any other implantable or external systems or devices fordiagnosing or treating the heart.

One problem faced by a cardiac rhythm management system treating certainventricular tachyarrhythmias (VT), including ventricular fibrillation(VF), by a countershock, is in distinguishing such potentially dangerousarrhythmias from other heart rhythms, such as a supraventriculartachyarrhythmia (SVT), for which delivery of a responsive countershockis inappropriate, painful, and potentially risky. Some examples of suchSVTs include atrial fibrillation (AF), atrial flutter, and sinustachyarrhythmia.

One technique used in an implantable cardiac rhythm management devicefor discriminating between ventricular and supraventriculartachyarrhythmias compares the shape (“morphology”) of each cardiaccomplex detected on an electrogram, during a period of high heart rate,to a template cardiac complex that was detected on the electrogramduring normal sinus rhythm experienced by an inactive patient. Adetected cardiac complex having a morphology similar to the template isdeemed indicative of an SVT. A detected cardiac complex having amorphology different from the template is deemed indicative of a VT.However, the present inventors have recognized that this determinationis confounded by the fact that some SVTs (e.g., “SVT with aberrancy”)also have a morphology different from the template obtained duringnormal sinus rhythm of an inactive patient. As a result, using the abovetechnique, such SVTs will instead be deemed indicative of VTs, resultingin the delivery of inappropriate countershocks. For these and otherreasons, the present inventors have recognized that there exists anunmet need for improved techniques of discriminating between SVTs andVTs.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are offered by way of example, and not by way oflimitation, and which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.Like numerals having different letter suffixes represent differentinstances of substantially similar components.

FIG. 1 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, portions of a cardiac rhythm managementsystem 100 using multiple morphology templates for discriminatingbetween heart rhythms.

FIG. 2 is a flow chart illustrating generally, by way of example, butnot by way of limitation, one method of distinguishing between heartrhythms using at least two morphological templates.

FIG. 3 is a flow chart illustrating generally, by way of example, butnot by way of limitation, one method of distinguishing betweensupraventricular tachyarrhythmia (SVT) and ventricular tachyarrhythmia(VT) using at least two morphological templates.

FIG. 4 is a flow chart illustrating generally, by way of example, butnot by way of limitation, a method of distinguishing between heartrhythms using two heart rate thresholds and, if the detected heart rateis between the two rate thresholds, using at least two morphologicaltemplates.

FIG. 5 is a flow chart illustrating generally, by way of example, butnot by way of limitation, a method of distinguishing between heartrhythms by using two heart rate thresholds and, if the detected heartrate exceeds both rate thresholds, using at least two morphologicaltemplates.

FIG. 6 is a signal diagram illustrating generally, by way of example,but not by way of limitation, a morphological template.

FIG. 7 is a signal diagram illustrating generally, by way of example,but not by way of limitation, comparison of a received complex to amorphological template, the received complex and the morphologicaltemplate aligned by an alignment feature, such as an R-wave peak, of anear-field signal.

FIG. 8 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of a system using at leastone sensor.

FIG. 9 is a graph of SIR (or SO) as a function of time, such as whereSIR (or SO) is used in acquiring and/or updating an “exercise template”T2 and/or a “resting template” T1.

FIG. 10 is a flow chart illustrating generally one example in which,among other things, an exercise template T2 is acquired only if there isinsufficient correlation to the resting template T1 during a period ofexercise.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the scope of the presentinvention. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined by the appended claims and their equivalents.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one. Furthermore, allpublications, patents, and patent documents referred to in this documentare incorporated by reference herein in their entirety, as thoughindividually incorporated by reference. In the event of inconsistentusages between this documents and those documents so incorporated byreference, the usage in the incorporated reference(s) should beconsidered supplementary to that of this document; for irreconcilableinconsistencies, the usage in this document controls.

This document discusses, among other things, systems, devices, andmethods that will be described in applications involving implantablemedical devices including, but not limited to, implantable cardiacrhythm management systems such as pacemakers,cardioverter/defibrillators, pacer/defibrillators, biventricular orother multi-site resynchronization or coordination devices, and drugdelivery systems. However, these systems, devices, and methods may beemployed in unimplanted devices, including, but not limited to, externalpacemakers, cardioverter/defibrillators, pacer/defibrillators,biventricular or other multi-site resynchronization or coordinationdevices, monitors, programmers and recorders, whether such devices areused for providing a diagnostic, a therapy, or both a diagnostic and atherapy.

FIG. 1 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, portions of a cardiac rhythm managementsystem 100 using multiple morphology templates for discriminatingbetween heart rhythms. In this example, system 100 includes animplantable cardiac rhythm management device 105 coupled to a heart 110by one or more intravascular or other leadwires 115. Each leadwire 115carries one or more electrodes sized and shaped to be disposed in orabout heart 110, such as for sensing intrinsic cardiac signals fromheart 110 and/or delivering electrical energy or other therapy to heart110. The illustrative example of FIG. 1 includes a tip electrode 120Adisposed at or near an apex of a right ventricle of heart 110,(optionally) a ring electrode 120B disposed slightly more proximally inthe right ventricle, a shock electrode 120C disposed even moreproximally in the right ventricle, and a superior vena cava (SVC) shockelectrode 120D located in or near portions of the right atrium andsuperior vena cava of heart 110. Device 105 includes ahermetically-sealed case 125, such as for carrying electronic componentstherein, and a header 130 attached thereto, such as for receiving one ormore leadwires 115. Device 105 may include additional electrodes, suchas case electrode 135A and header electrode 135B, such as for unipolarsensing or therapy energy delivery.

In the illustrative example of FIG. 1, device 105 includes a cardiacsignal sensing circuit 140. In this example, cardiac signal sensingcircuit 140 includes a near-field sense amplifier 145A and a far-fieldsense amplifier 145B. In one example of a bipolar sensing configuration,near-field sense amplifier 145A is coupled to electrodes locatedrelatively close to each other, such as electrodes 120A and 120C, forsensing a near-field cardiac signal; far-field sense amplifier 145B iscoupled to electrodes located relatively farther from each other, suchas electrodes 120C and 135A (or, alternatively, electrode 120C and theparallel combination of electrodes 120D and 135A), for sensing afar-field cardiac signal that includes information from a greater regionof cardiac tissue. The resulting sensed near-field cardiac signalprovided at node 150A typically includes relatively sharply definedcardiac complexes corresponding to intrinsic heart chamberdepolarizations. Such relatively sharply defined cardiac complexestypically allow heart rate to be relatively easily discerned by ratedetector 155, which provides an indication of the heart rate betweensensed near-field cardiac complexes at node 155. The resulting cardiaccomplexes of the sensed far-field cardiac signal, provided at node 150B,typically exhibit some differences in morphology during different heartrhythms. Therefore, such differences in morphology of sensed far-fieldcardiac complexes are particularly useful for discriminating betweendifferent heart rhythms.

In FIG. 1, device 105 includes a buffer or other memory storage 160 fornear-field and/or far-field data associated with recently-detectedcardiac complexes. In this example, device 105 also includes memorystorage for data associated with two or more morphological templates,such as a first morphological template 165A and for a secondmorphological template 165B, which are obtained from heart 110 underdifferent conditions from each other, as explained further below.(Templates 165A-B may, in one example, be implemented entirely insoftware, such as by storing corresponding morphology-defining sets ofmorphological features). In the illustrated example, device 105 alsoincludes a correlation module 170, which distinguishes between at leasttwo different rhythm states by comparing and correlating the morphologyof at least one recently received cardiac complex to at least one of thestored first and second morphological templates 165A-B. In certainexamples, as explained below, correlation module 170 also uses the heartrate at node 155 in discriminating between different rhythm states.

An output of correlation module 170 provides, at node 175, an indicationof the particular rhythm state, if any, obtained as a result of thecomparison and correlation. In one example, correlation module 170declares whether a detected arrhythmia is a supraventriculartachyarrhythmia (SVT) or a ventricular tachyarrhythmia (VT), andprovides an indication of the same to shock control module 180. Shockcontrol module 180 provides one or more triggering signals controllingdelivery of a defibrillation countershock to heart 110 by shock circuit185, such as a shock delivered between shock electrodes 120C-D, forexample. In one example, shock control module 180 operates to inhibitdelivery of a defibrillation shock if correlation module 170 declares anSVT, and operates to trigger delivery of a defibrillation shock ifcorrelation module 170 declares a VT.

In the example of FIG. 1, system 100 also includes a programmer or otherremote user interface 190, which is configured to be wirelesslycommunicatively coupled to a communication circuit 195 of device 105. Inone example, remote user interface 190 allows a user to provide inputinformation that is used in the distinguishing between heart rhythmstates using multiple morphological templates. In another example,remote user interface 190 outputs information to the user relevant tothe distinguishing between heart rhythm states using multiplemorphological templates, by device 105.

FIG. 2 is a flow chart illustrating generally, by way of example, butnot by way of limitation, one method of distinguishing between heartrhythms using at least two morphological templates. In the example ofFIG. 2, a heart depolarization is detected at 200, such as by senseamplifiers 145A and/or 145B. At 205, a heart rate, HR, is compared to apredetermined tachyarrhythmia rate threshold R1 (for an illustrativeexample, R1=145 beats per minute). HR>R1 indicates the presence of atachyarrhythmia. HR≦R1 indicates the absence of a tachyarrhythmia. Inone example, the heart rate used for the comparison is measured betweenthe detected heart depolarization and an immediately preceding heartdepolarization. In another example, an average heart rate over severalsuccessive pairs of heart depolarization is used instead.

If HR>R1, at 205, indicating the presence of a tachyarrhythmia, then adetermination is made that further classifies the rhythm state, asdescribed below. At 210, a morphology of the detected depolarizationcomplex is compared to the first morphological template, T1, such asstored at 165A. In one example, this includes determining a degree ofcorrelation between the morphologies of the detected complex and T1,comparing the degree of correlation to a predetermined threshold, anddeclaring a match if the correlation exceeds that predeterminedthreshold. In another example, this comparison includes correlatingseveral (e.g., successive) detected depolarization morphologies to thefirst morphological template T1, and requiring that a predeterminedpercentage of the detected depolarizations be sufficiently correlated toT1 before a match is declared. In either case, if sufficient correlationexists to declare a match, then, at 215, a first rhythm state isdeclared.

Otherwise, at 220, a determination is made as to whether a secondmorphological template, T2, was previously stored at 165B. If no T2 hasbeen stored, then, at 225, a second rhythm state is declared. Otherwise,at 230, a morphology of the detected depolarization complex is comparedto the second morphological template, T2. In one example, this includesdetermining a degree of correlation between the morphologies of thedetected complex and T2, comparing the degree of correlation to apredetermined threshold (which may be different than that for T1), anddeclaring a match if the correlation exceeds that predeterminedthreshold. In another example, this comparison includes correlatingseveral (e.g., successive) detected depolarization morphologies to T2,and requiring that a predetermined percentage of the detecteddepolarizations be sufficiently correlated to T2 before a match isdeclared. In either case, if sufficient correlation exists at 230 todeclare a match, then, at 235, a first rhythm state is declared.Otherwise, at 240, a second rhythm state is declared.

In a further example, the particular rhythm state obtained, as discussedabove, is used as a control input affecting the delivery of electricalenergy or other therapy to heart 110. In the example of FIG. 1, if thefirst rhythm state was declared at 215 or 235, then antitachyarrhythmiashock delivery is inhibited at 245. If the second rhythm state wasdeclared at 225 or 240, then antitachyarrhythmia shock delivery istriggered at 250.

EXAMPLES OF MORPHOLOGICAL DISCRIMINATION BETWEEN RHYTHM STATES Example 1

In a first example, first morphological template T1 corresponds tonormal sinus rhythm obtained from a subject's heart 110 while thesubject is resting or relatively inactive—and no tachyarrhythmia ispresent. Second morphological template T2 corresponds to normal sinusrhythm obtained from the subject's heart 110 while the subject isexercising or relatively active—and no ventricular tachyarrhythmia (VT)is present. For example, for acquiring and storing T2, the subject canbe placed on a treadmill and an appropriate template depolarizationcomplex acquired. In this example, a physician independently verifies(e.g., using a surface ECG and/or electrogram signals) that no VT waspresent during acquisition of T2. As an alternative to placing thesubject on the treadmill, the physician may program device 105 todeliver atrial pacing pulses at a high rate, e.g., using an atrialleadwire; again, a physician verifies that no VT was present during thisacquisition of T2.

In this example, as illustrated in the flow chart of FIG. 3, the firstrhythm state is declared a supraventricular tachyarrhythmia (SVT), whichresults in inhibiting antitacharrhythmia shock delivery at 245, and thesecond rhythm state is declared a ventricular tachyarrhythmia (VT),which results in triggering antitachyarrhythmia shock delivery at 250.The present inventors have recognized that the use of a resting templateT1 and an exercise template T2 accounts for morphological differencesarising during exercise that are not indicative of VT. Using exercisetemplate T2 adds another non-VT condition for which shock delivery isinhibited. This improves the specificity of deliveringantitachyarrhythmia shock therapy for VTs, but not SVTs. For example,subjects experiencing left or right bundle branch block (BBB) inducedduring exercise will benefit from the additional specificity of using amorphological comparison of a detected depolarization complex to anexercise morphology template T2 as well as a resting morphology templateT1. Similarly, other subjects experiencing left or right bundle branchblock (BBB) mitigated during exercise will also benefit from theadditional specificity of using a morphological comparison of a detecteddepolarization complex to an exercise morphology template T2 as well asa resting morphology template T1. These are merely illustrative examplesof physiological conditions for which additional antitachyarrhythmiatherapy delivery specificity is obtained; other physiological conditionsexist that will also obtain increased specificity.

Example 2

In a second example, first morphological template T1 corresponds tonormal sinus rhythm obtained from a subject's heart 110 while thesubject is resting or relatively inactive, and no tachyarrhythmia ispresent. Second morphological template T2 corresponds tosupraventricular tachyarrhythmia (SVT) rhythm obtained from thesubject's heart 110 while no accompanying ventricular tachyarrhythmia(VT) is present. In one example, such an SVT may be induced by aphysician in an electrophysiology (EP) lab; the physician independentlyverifies (e.g., using a surface ECG and/or electrogram signals) that noVT was present during acquisition of T2 during the SVT. In anotherexample, such SVT template data is obtained from historical electrogramdata obtained from the subject and stored by device 105; the physicianindependently verifies (e.g., using the stored electrogram signals) thatno VT was present during acquisition of T2 during the stored SVTepisode. Then, as discussed above with respect to FIG. 3, device 105uses morphological comparisons of detected cardiac complexes to T1 andT2 to discriminate between SVT and VT, and adjust antitachyarrhythmiatherapy delivery accordingly.

Example 3

In a third example, as illustrated in the flow chart of FIG. 4, theheart rate is compared to more than one threshold. In the example ofFIG. 4, if the heart rate exceeds a first threshold R1 at 205, it isthen compared at 400 to a second (higher) rate threshold R2 (for anillustrative example, R2=165 beats per minute). If, at 400, HR>R2, then,at 405, VT is declared. An antitachyarrhythmia shock is then triggeredat 250. Otherwise if, at 400, R2>HR>R1, the process flow continues at210 as discussed above with respect to FIGS. 2 and 3. In the example ofFIG. 4, therefore, an extremely high detected rate triggers adeclaration of VT and bypasses any comparison of a morphology of adetected depolarization complex to multiple morphological templates.

Example 4

In a fourth example, as illustrated in the flow chart of FIG. 5, theheart rate is used to determine whether a morphological comparison ismade to more than one morphological template. In FIG. 5, if thedepolarization is not correlated to T1 at 210, then at 500 the heartrate is compared to a second (higher) rate threshold R2 (for anillustrative example, R2=165 beats per minute). If, at 500, HR>R2, thena further comparison is made at 230 to T2, as discussed above withrespect to FIGS. 2 and 3. Otherwise, if, at 500, R2>HR>R1, then, at 225,VT is declared, as discussed above with respect to FIGS. 2 and 3.

In all of the above examples, it is understood that morphologicalcomparisons to more than two morphological templates (e.g., 3 templates,4 templates) are also possible, and are included as additionalembodiments of the systems, devices, and methods described in thisdocument. In one such example, template T2 includes a plurality ofmultiple morphological templates to which a morphology comparison ismade. Moreover, additional comparisons of heart rate to more than twothreshold values are also possible and included as additionalembodiments of the systems, devices, and methods described in thisdocument. As a result, other embodiments may be capable ofdistinguishing between more than two different heart rhythm states(e.g., 3 heart rhythm states, 4 heart rhythm states, etc.), andaccordingly adjusting therapy using such additional classification intoseveral different rhythm states.

Also, because a particular subject's cardiac complex morphology maychange over time (e.g., because of the effect of a drug beingadministered, or a change in the subject's heart condition), themultiple templates are typically updated occasionally or periodically.In one example, acquiring or updating a template is typically performedunder the same or similar conditions to those conditions for which thecorrelation is performed. For example, where the templates T1 and T2 aredifferentiated by heart rate, in one example, device 105 uses ratedetector 155 for automatically acquiring and/or updating the templatesT1 and T2 under their corresponding heart rate conditions. In anotherexample, however, device 105 uses a different sensor for acquiringand/or updating an “exercise template” T2, as discussed below.

EXAMPLES OF OPERATION OF THE CORRELATION MODULE

FIG. 6 is a signal diagram illustrating generally, by way of example,but not by way of limitation, one example of a morphological template600, such as T1 or T2, obtained from the far-field signal at 150B. Inthis example, template 600 includes a collection of eightmorphology-defining features 605A-H extracted from the far-field signal.In this example, an R-wave peak on a corresponding near-field signal at150A, is used as an “alignment feature” of the template 600. Before thestored template 600 is later correlated to a received far-field cardiaccomplex, the point on the received far-field cardiac complex that alignsto its corresponding near-field R-wave peak (or other selected alignmentfeature) is used to “align” the received far-field cardiac complex tothe template 600. More particularly, the template 600 is time-shiftedsuch that the time coordinate of the R-wave peak of the near-fieldsignal associated with the received far-field complex 700 beingcorrelated to the template 600, as illustrated in the signal diagram ofFIG. 7. Template 600 stores the times and amplitudes of each of theeight features 605A-H for comparison to a received far-field complex700, such that the received far-field complex 700 can be classified intoa rhythm state.

One illustrative example of the features 605A-H is disclosed in JaehoKim and et al. U.S. Pat. No. 6,889,079, entitled “METHOD AND SYSTEM FORCHARACTERIZING SUPRAVENTRICULAR RHYTHM DURING CARDIAC PACING,” which isincorporated herein by reference in its entirety, including itsdisclosure of obtaining eight features by first identifying five initialfeatures, and then identifying three additional features determined atpoints between certain ones of the five initial features.

The received far-field cardiac complex 700 is sampled at the same times(relative to the alignment feature) as the features 605A-H in template600, yielding comparison features 705A-H. In one example, correlationmodule 170 computes a feature correlation coefficient (FCC) using theamplitude (x_(i)) of each of the template features 605A-H and theamplitude (y_(i)) of the received far-field cardiac complex at thesesame times 705A-H relative to the alignment feature, as illustrated byEquation 1, below:

$\begin{matrix}{{FCC} = \frac{\left( {{8{\sum\limits_{i = 1}^{8}{x_{i}y_{i}}}} - {\left( {\sum\limits_{i = 1}^{8}x_{i}} \right)\left( {\sum\limits_{i = 1}^{8}y_{i}} \right)}} \right)^{2}}{\left( {{8{\sum\limits_{i = 1}^{8}x_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{8}x_{i}} \right)^{2}} \right)\left( {{8{\sum\limits_{i = 1}^{8}y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{8}y_{i}} \right)^{2}} \right)}} & (1)\end{matrix}$In one example, the FCC computed in Equation 1 is compared to apredetermined threshold value to determine whether the receivedfar-field cardiac complex 700 is correlated to the template 600. In oneexample, if this comparison indicates that the received complex 700 isuncorrelated to the template 600, then a second heart rhythm beat (e.g.,VT beat) is declared. If 8 or more of the last 10 beats is uncorrelated,then correlation module 170 declares a second heart rhythm state (e.g.,VT is declared). In one example, such correlation techniques are appliedfor comparison to both templates T1 and T2, such as at 210 and 230 ofFIG. 2.

Other techniques for comparing received complexes to morphologicaltemplates can be substituted for the illustrative example discussedabove. This document incorporates herein by reference the entiredisclosure of Hsu et al. U.S. Pat. No. 6,308,095, entitled “SYSTEM ANDMETHOD FOR ARRHYTHMIA DISCRIMINATION,” which is assigned to CardiacPacemakers, Inc., including incorporation of its disclosure ofclassifying cardiac complexes using morphological features. Thisdocument also incorporates herein by reference the entire disclosure ofMarcoveccio U.S. Pat. No. 6,223,078, entitled “DISCRIMINATION OFSUPRAVENTRICULAR TACHYCARDIA AND VENTRICULAR TACHYCARDIA EVENTS,” whichis assigned to Cardiac Pacemakers, Inc., including incorporation of itsdisclosure of classifying cardiac complexes using morphologicalfeatures. This document incorporates herein by reference the entiredisclosure of Hsu et al. U.S. Pat. No. 6,275,732, entitled “MULTIPLESTAGE MORPHOLOGY-BASED SYSTEM DETECTING VENTRICULAR TACHYCARDIA ANDSUPRAVENTRICULAR TACHYCARDIA,” which is assigned to Cardiac Pacemakers,Inc., including incorporation of its disclosure of classifying cardiaccomplexes using morphological features. This document also incorporatesherein by reference the entire disclosure of Marcoveccio U.S. Pat. No.6,312,388, entitled “METHOD AND SYSTEM FOR VERIFYING THE INTEGRITY OFNORMAL SINUS RHYTHM TEMPLATES,” which is assigned to Cardiac Pacemakers,Inc., including incorporation of its disclosure of classifying cardiaccomplexes using morphological features and updating templates. Thisdocument incorporates herein by reference the entire disclosure of Hsuet al. U.S. Pat. No. 6,266,554, entitled “SYSTEM AND METHOD FORCLASSIFYING CARDIAC COMPLEXES,” which is assigned to Cardiac Pacemakers,Inc., including incorporation of its disclosure of classifying cardiaccomplexes using morphological features. This document incorporatesherein by reference the entire disclosure of Hsu et al. U.S. Pat. No.6,449,503, entitled “CLASSIFICATION OF SUPRAVENTRICULAR AND VENTRICULARCARDIAC RHYTHMS USING THE CROSS CHANNEL TIMING ALGORITHM,” which isassigned to Cardiac Pacemakers, Inc., including incorporation of itsdisclosure of classifying cardiac complexes using morphologicalfeatures. This document incorporates herein by reference the entiredisclosure of Sweeney et al. U.S. Pat. No. 6,684,100, entitled“CURVATURE BASED METHOD FOR SELECTING FEATURES FROM ANELECTROPHYSIOLOGIC SIGNALS FOR PURPOSE OF COMPLEX IDENTIFICATION ANDCLASSIFICATION,” which is assigned to Cardiac Pacemakers, Inc.,including incorporation of its disclosure of classifying cardiaccomplexes using morphological features and curvatures. This documentincorporates herein by reference the entire disclosure of Lovett U.S.Pat. No. 6,434,417, entitled “METHOD AND SYSTEM FOR DETECTING CARDIACDEPOLARIZATION,” which is assigned to Cardiac Pacemakers, Inc.,including incorporation of its disclosure of classifying cardiaccomplexes using morphological features and frequency components. Thisdocument incorporates herein by reference the entire disclosure ofSweeney et al. U.S. Pat. No. 6,526,313, entitled “SYSTEM AND METHOD FORCLASSIFYING CARDIAC DEPOLARIZATION COMPLEXES WITH MULTI-DIMENSIONALCORRELATION,” which is assigned to Cardiac Pacemakers, Inc., includingincorporation of its disclosure of classifying cardiac complexes usingmorphological features and multidimensional correlation.

In the above discussion of FIGS. 1-7, the systems and methods utilized aheart rate (e.g., at node 155) that was represented as being obtainedfrom a rate detector 155 that extracts heart rate from a near fieldsignal obtained from cardiac electrodes. This heart rate was also usedfor comparing to various rate thresholds (see, e.g., 205 of FIG. 2, 400of FIG. 4, etc.). However, obtaining heart rate from cardiac electrodesfor distinguishing between heart rhythm states may, in certainconditions, be affected by the arrhythmias being distinguished, noisycardiac signals, etc. Therefore, it may be desirable to either validatesuch sensed heart rate information obtained from an electrogram, or,alternatively, to use a different indication of heart rate.

FIG. 8 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of a system 100 using atleast one sensor 800. In the example of FIG. 8, sensor 800 may includean accelerometer, a minute ventilation sensor, or the like, providing anindication of patient activity. In one example, this indication ofpatient activity is provided as a sensor output (SO) at node/bus 802, tocorrelation module 170. The SO is positively correlated to a patient'sactivity (for an activity sensor) or metabolic need (for a metabolicneed sensor); a larger value of SO corresponds to a higher activitylevel (or metabolic need). In a further example, the indication ofpatient activity is provided as a sensor-indicated rate (SIR), atnode/bus 802, to correlation module 170. The SIR represents a computedheart rate deemed appropriate for the patient, based on activity and/ormetabolic need information obtained from the SO of sensor 800. The SIRis also positively correlated to a patient's activity (for an activitysensor) or metabolic need (for a metabolic need sensor); a larger valueof SIR corresponds to a higher activity level (or metabolic need).Numerous techniques known in the art (e.g., using rate-response curves)are available for mapping the SO to the SIR.

In one example, SIR (or SO) is used in acquiring and/or updating an“exercise template” T2 and/or a “resting template” T1, as illustratedgenerally in the graph of FIG. 9, which depicts SIR (or SO) as afunction of time. In FIG. 9, the exercise template T2 is acquired (orupdated) at point 900, after the SIR (or SO) has exceeded acorresponding exercise threshold S1 for a predetermined period of timeΔt1. A resting template T1 is acquired (or updated) at point 905, afterthe SIR (or SO) has fallen below the corresponding exercise threshold S1for a predetermined period of time Δt2. Alternatively, two differentvalues of the activity threshold S1 (e.g., S1A and S1B) are used fortriggering the respective time periods (Δt1 and Δt2) after which therespective exercise and resting templates are obtained. In anotherexample, these two different values of the activity threshold (e.g., S1Aand S1B) trigger the obtaining of the respective exercise and restingtemplates T2 and T1, without requiring the SIR or SO to be above orbelow such threshold values for a period of time.

FIG. 10 is a flow chart illustrating generally one example in which,among other things, an exercise template T2 is acquired only if there isinsufficient correlation to the resting template T1 during a period ofexercise. In the example of FIG. 10, a heart depolarization is detectedat 1000. At 1005, the SIR (or SO) and HR are monitored; if the SIRexceeds a predetermined threshold S1, and the HR exceeds a predeterminedthreshold R1, then at 1010 a determination is made as to whether thedetected depolarization is sufficiently correlated to the restingtemplate T1. If, at 1000, the detected depolarization is notsufficiently correlated to the resting template T1, and there is not astored exercise template T2 at 1015, then at 1020, an exercise templateT2 is acquired. At 1015, if there is not a stored exercise template T2,then at 1025 a determination is made as to whether the detecteddepolarization is sufficiently correlated to the existing exercisetemplate T2. If not, then exercise template T2 is updated at 1020. In analternative embodiment, multiple exercise templates T2 are acquired at1020 if the depolarization is not sufficiently correlated to any of theexisting exercise templates at 1025.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, aspects of theabove-discussed examples may be used in combination with each other.Many other embodiments will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention should,therefore, be determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled.Moreover, the terms “first,” “second,” “third,” etc. are used merely aslabels, and are not intended to impose numeric requirements on theirobjects.

1. A machine-readable medium including instructions that, when performedby a machine, cause the machine to: obtain a first referencemorphological template from a subject's heart in the absence of anarrhythmia, while the subject is resting or inactive; obtain a secondreference morphological template from the heart under a conditiondifferent from that of the first morphological template; obtain at leastone current cardiac complex; determine a degree of correlation betweenthe at least one cardiac complex and at least one of the first andsecond morphological templates; and declare one of first and secondrhythm states at least in part using the determined degree ofcorrelation.
 2. The machine-readable medium of claim 1, furtherincluding instructions that cause the machine to determine whether todeliver or inhibit a shock to a heart at least in part using which ofthe first and second rhythm states was declared.
 3. The machine-readablemedium of claim 1, in which the instructions to obtain the secondmorphological template occur under a condition in which the subjectmanifests at least one activity indicator value that is higher than theat least one activity indicator value that occurred during the obtainingthe first morphological template.
 4. The machine-readable medium ofclaim 1, in which the instructions to obtain the second morphologicaltemplate occur under a condition in which the subject manifests anarrhythmia.
 5. The machine-readable medium of claim 4, in which theinstructions to obtain the second morphological template occur under acondition in which the subject manifests an induced arrhythmia.
 6. Themachine-readable medium of claim 1, in which the instructions to obtainthe second morphological template occur during high-rate atrial pacing.7. The machine-readable medium of claim 1, in which the instructions toobtain the second morphological template occur during controlledphysical exertion.
 8. The machine-readable medium of claim 1, in whichthe instructions to obtain the second morphological template occur undera condition in which the subject manifests a supraventriculartachyarrhythmia.
 9. The machine-readable medium of claim 1, furthercomprising instructions to recurrently update at least one of the firstand second morphological templates.
 10. The machine-readable medium ofclaim 8, in which the episode of the supraventricular tachyarrhythmia isobtained from historical electrogram data.
 11. A method comprising:obtaining a first reference morphological template from a subject'sheart in the absence of an arrhythmia, while the subject is resting orinactive; obtaining a second reference morphological template from theheart during high-rate atrial pacing; obtaining at least one currentcardiac complex; determining a degree of correlation between the atleast one cardiac complex and at least one of the first and secondmorphological templates; and declaring one of a first and second rhythmstate at least in part using the determined degree of correlation. 12.The method of claim 11, further including determining whether to deliveror inhibit a shock to a heart at least in part using which of the firstand second rhythm states was declared.
 13. A method comprising:obtaining a first reference morphological template from a subject'sheart in the absence of an arrhythmia, while the subject is resting orinactive; obtaining a second reference morphological template from theheart during exercise; obtaining at least one current cardiac complex;determining a degree of correlation between the at least one cardiaccomplex and at least one of the first and second morphologicaltemplates; and declaring one of a first and second rhythm state at leastin part using the determined degree of correlation.
 14. The method ofclaim 13, further including determining whether to deliver or inhibit ashock to a heart at least in part using which of the first and secondrhythm states was declared.
 15. A method comprising: obtaining a firstreference morphological template from a subject's heart in the absenceof an arrhythmia, while the subject is resting or inactive; obtaining asecond reference morphological template, in which the second template isobtained from historical electrogram data of an episode ofsupraventricular tachyarrhythmia; obtaining at least one current cardiaccomplex; determining a degree of correlation between the at least onecardiac complex and at least one of the first and second morphologicaltemplates; and declaring one of a first and second rhythm state at leastin part using the determined degree of correlation.
 16. The method ofclaim 15, further including determining whether to deliver or inhibit ashock to a heart at least in part using which of the first and secondrhythm states was declared.
 17. A system comprising: an electrogramsensing circuit, configured to receive first and second intrinsiccardiac signals, the electrogram sensing circuit configured to providecardiac complexes, each cardiac complex having a morphology, and anindication of heart rate obtained from the first intrinsic cardiacsignal; a sensor, including a sensor output indicative of activity ormetabolic need of the subject; a stored first reference morphologicaltemplate, coupled to the electrogram sensing circuit, the stored firstmorphological template obtained from the heart in the absence of anarrhythmia while the subject is resting or inactive; a stored secondreference morphological template, coupled to the electrogram sensingcircuit and the sensor, the stored second morphological templateobtained from the heart under a different sensor output condition fromthat of the first morphological template; a correlation module, coupledto the electrogram sensing circuit and the first and secondmorphological templates, the correlation module configured to declare anindication of one of a first and a second rhythm state, includingdiscriminating between the first and second rhythm states using acomparison of a morphology of at least one current cardiac complex ofthe second intrinsic cardiac signal to at least one of the first andsecond morphological templates, the at least one of the first and secondmorphological templates selected using the indication of heart rateprovided by the electrogram sensing circuit; and a template updatingcontrol module, coupled to the second morphological template and thecorrelation module, the template updating control module configured toprovide an updated exercise template to the correlation module if: (a) afirst exercise template is not available to the correlation module or(b) an available exercise template is insufficiently correlated to theat least one cardiac complex of the intrinsic cardiac signal.
 18. Thesystem of claim 17, further comprising: a shock circuit, configured tobe coupled to at least one electrode for delivering a shock to theheart; and a shock control module, coupled to the correlation module andthe shock circuit, the shock control module configured to trigger ashock if the correlation module declares the indication of the secondrhythm state, and the shock control module configured to inhibit a shockif the correlation module declares the indication of the first rhythmstate.
 19. The system of claim 17, further comprising: a firstelectrode, sized and shaped for being implanted in or near a subject'sheart, the first electrode configured for sensing a heart rate fromcardiac complexes; a second electrode, sized and shaped for beingimplanted in or near the heart, the second electrode configured forsensing a morphology of a cardiac complex.
 20. A method comprising:obtaining a first reference morphological template from a subject'sheart in the absence of an arrhythmia, while the subject is resting orinactive; obtaining a second reference morphological template from theheart under a condition different from that of the first morphologicaltemplate; obtaining at least one current cardiac complex; determining adegree of correlation between the at least one cardiac complex and atleast one of the first and second morphological templates; declaring oneof a first and second rhythm state at least in part using the determineddegree of correlation; and updating the second morphological template ifthe cardiac complex is not sufficiently correlated to at least one ofthe first and second morphological templates.
 21. A system comprising:means for obtaining a first reference morphological template from asubject's heart in the absence of an arrhythmia, while the subject isresting or inactive; means for obtaining a second referencemorphological template from the heart under a condition different fromthat of the first morphological template; means for obtaining at leastone current cardiac complex; means for determining a degree ofcorrelation between the at least one cardiac complex and at least one ofthe first and second morphological templates; means for declaring one offirst and second rhythm states at least in part using the determineddegree of correlation; and means for updating at least one of the firstand second morphological templates.